1. Field of the Invention
This invention relates to a diesel engine which assures excellent engine performance and can purify exhaust gas satisfactorily.
2. Description of the Related Art
The diesel engine has to meet very strict requirements for much cleaner exhaust gas. A wide assortment of measure have been proposed and devised for this purpose. Various control systems have been proposed and put into practical use to decrease nitrogen oxides (NOx) in the exhaust gas, and hydrocarbon in particulates and soot respectively.
A variable swirl control system has been proposed to form air swirls in a combustion chamber, for thereby efficiently forming an air-fuel mixture to burn the fuel completely and decrease the particulate and soot. An exhaust gas recirculation (EGR) control system recirculates part of exhaust gas to the combustion chamber according to the working conditions of the engine, for decreasing the concentration of oxide, lowering combustion temperature, and suppressing NOx. A timing control system controls the fuel injection timing to reduce NOx and particulates. Furthermore, efforts have been made to devise shapes of fuel injection units and combustion chambers. The exhaust gas discharged via the foregoing control systems are further filtered by an exhaust gas purifier located in a scavenge passage.
Any of the foregoing control systems however cannot independently cope with all the problems of the exhaust gas. These control systems have advantages and disadvantages. To purify the exhaust gas by a single control system, such a control system inevitably becomes more complicated and more expensive. Specifically, it is not possible at all to have a single control system purify the exhaust gas of a vehicle engine whose working condition changes incessantly. Conventional control systems are considered acceptable when they purify the exhaust gas to a certain preferable degree. Usually, to emit cleaner exhaust gas, engine performance is somewhat sacrificed. For instance, suppression of NOx results in the increase of concentration of smoke and hydrocarbon, and the decrease of the engine efficiency. Therefore, the foregoing control systems make it difficult to meet the new requirements for much cleaner exhaust gas so as to prevent air contamination.